Hypergolic fuel containing mercaptals or mercaptols



. I 2,940,250 HYPERGOLIC FUEL CONTAINING MERCAPTALS R MERCAPTOLS Manuel A..Pino, Richmond, and Paul C. Condit, Berkeley,

Calif., assignors to California Research. Corporation,

San Francisco, Calif., a corporation of Delaware No Drawing. Filed Jan. 27, 1954, Ser. No. 406,617

. 2 Claims. (Cl. Gil-35.4)

This invention relates to fuel compositions which are characterized by self-ignition when mixed with powerful oxidizing agents and to a method for initiating and maintaining combustion through the use of these fuels.

Self-igniting compositions of various kinds have for many years found use in the construction of assorted tools of war for use against personnel and property. More recently it, has beenrecognized that self-igniting compositions are useful as propellants for rockets, guided missiles and aircraft powered by jet or thrust engines. Further, self-igniting compositionsare -useful in initiating combustion in areas in which the usual methods of initiating combustion can be employed only with great difiiculty and inconvenience if at all. For example, self-i lliiing compositions may be employed to initiate combustion in depleted oil-bearing formations to effect secondary recovery of residual oil in the formation by burning a part of the residual oil and recovering any substantial portion of the remaining residual oil by evaporation, cracking and gas drive of the combustion gasesat a producing well. I

Hypergolic fuels are useful in the operation-of thrust engines. Hypergolic fuels may be generally described as 2,940,256 Patented June 14, 1960 'bustion chamber of the engine to form an ignited 'propulsive charge and ejecting the gases formed by burning charge from the chamber at high velocity. 1 1

It has further been found that a mixture consisting of essentially about 75% by weight of isopropylidene 'diethyl mercaptol and about by weight ethylidene-diethyl mercaptal possess an'extraordinary combination of physical andchemical properties which make the mixture extremely valuable as the reducing component of a hypergolic fuel.

The mercaptals and mercaptols of the invention are readily prepared by the reaction of a mercaptanwith an aldehyde or ketone according to the following equation:

g 2Rsrr+R'ii-R"- R'-o-R(q-rno is an organic radical,

is either an alkyl radical or hydrogen. A single mercaptan or a mixture of lower mercaptans may be employed in the reaction. Usually a smallamount of an acid catalyst mixtures of a reducing component and an oxidizing component. The individual components of the hypergolic fuel are not self-igniting, but when the two components are mixed there is a short period during which relatively slow reaction of the two components apparently occurs,

and this relatively slow reaction is followed by spontaneous ignition of the mixture. Practically all hypergolic j mixtures have short ignition delay periods substantially less than 1 second and preferably not more than about 50 milliseconds. The reducing component of a hypergolic fuel should hav certain properties, as follows: It should ignite" spontaneously upon being combined with the "selected"oxidizing agent; the ignition delay period should be short, preferablynot exceeding 50 milliseconds; it should have a raw freezing-point so that it can be pumped to the combustion chamber at the low temperatures prevailing at high altitudes; it should have a high boiling point to facilitate handling and increase its safety in use; itshould bechemically' stable in storage; it should have a relatively low viscosity at low temperatures; it should form gaseous oxidation products and the average molecular Weight of these products should be as low as possible in order that the specific impulse (pounds thrust/ pounds propellant/second) be high. In addition to the foregoing properties, it is desirable that self-ignition of the mixture of the reducing component and the oxidizing agent be rapidover a wide temperature range; that the reducing component have a high density so that the volume of fuel tanks may be reduced; that thematerial be non-toxic and easy to handle; that it have a high heat of combustion; and that it be non-corrosive.

It has now been found that a fire can be started by rapid auto-ignition by mixing at least one material of the group consisting of mercaptals and mercaptols containing 3 to 12 carbon atoms per molecule with fuming nitric acid.

" Inone embodiment of the invention a thrust engine is operated by passing a material of the group consisting ofmercaptals and mercaptols containing 3 to 12 carbon atoms per'molecule and fuming nitric acid into the com- Anumber of mercaptals and mercaptols were prepared by reacting mercaptans .with .carbonyl compounds, and these materials were tested for self-ignition by a simple drop test. In this test, the mercaptals and mercaptols weredropped from a capillary pipette into a POOLQf concentrated (95-l00%) nitric'a'cid contained in the bottom of a Pyrex tube. As standard quantities, 0.13 cc. of the organic compound and 0.40 cc. of nitric acid are used.

Spontaneous ignition is indicated by a flash of light in the bottom of the Pyrex tube. Occasionally when testing a fuel and it is found that ignition does not occur, activity of a fuel may be indicated by the evolution of fumes and heat from the mixture. The drop tests are conducted at room temperature. The results of drop tests of a number of mercaptalsand' mercaptols in the manner above described are reported'in the following Table I. Instead of setting out the chemical names of the mercaptals and mercaptols, their composition is indicated by listing the carbonyl component and the mercaptan component from which they were made according to the above equation.

TABLE I Drop tests of mercaptals and mercaptols with concentrated HNO Carbonyl Component Mercaptan Component Furfural. do Do. 7 Do t-Butyl Mercaptam. Do. I Butyraldehyden n-Deoyl Mercaptan. no igniitlon, slow 1 react on. Crotonaldehyde... Mixed C2-C Merignited}.

captan. Furfural .do Do. Crotonaldehyde... A1ly1Mereaptan... Do. Furfural o Do. Acetaldehyde Methyl Mercaptan. Do. Acetone d Do. Formaldehyde Do. Acetaldehyde- Do. Formaldehyde Do. Acetaldehyde. Do. Do. Do. Acetone ,,Do. Methyl Ethyl Ketone Do. Acetaldehyde ta Do. Aeroleln Ethyl Mercaptan Do. Benzaldehyde Methyl Mercaptam. Do.

From the above table it is clear that mercaptals and mercaptols containing .3 to 12 carbon atoms per molecule spontaneouslyignite When mixed with concentrated nitric acid.

ethyl-.- hcetaldehyde;

V 'InJorder that a hypergolic mixture be useful as a thrust engine fuel, it. is. not sufiicient that. the mixture. undergo.

" lnitibni delay; neriod'hel obtained atlo'w temperatures;

for example. 7-40. toj--70 15., since temperatures in range. are commonlyencountered in high altitude operation: of aircraft powered. by thrust, engines. The following: Table. II. shows ignition delay: periods of a number; oif .mercaptals. and mercaptols at. low temperature.

The ignition delay is measured by mixing themercaptal It will be noted that the blend has a" substantially lower visocity than either of the individual components at 70 F. and that the freeze point of the blend is substantially lower than the freeze" point of the mercaptol component. j i if c j The ignition'delay periods of the individual mercaptol and mercapt'al and of blends of'the; two materialsare illustratedin the following Table. IV:

Ignition. delays of various blendsof ethyl mercaptal of acetaldehyde 'amtietliyl merca'ptol of aceton'e'at 70 F. using red fumie nitrie acid as theoxidizer do 80% HNO320% Ethane Sulfonie Acid l0 80%fiHNOz-20% Methane Sulfonlc Acid -40 i 9 80% HNOr20% Ethane Sulfonic Acidor; mercaptol with'the oxidizer; in a specially constructed c Q m ge; j cylinderso arranged that at the instant of mixing the 131 (v01. Ifereent) I gen organic compound .andthe .oxidizer an; electrode .is 7 p y" grounded transmitting a pulse to an electronic timer to Me c n Memen o) commence the measurements of the delay. Soon after i p p v I the acid contacts the organicmaterial the ignition takes 28' g 5} place. Ionization caused by the flamegrounds the tubu- 75 26.,5' lar electrode to the Walls of the oxidizer cylinder trans 7 9 p v f mitting a second pulse to the electronic timer to corn- V plete'the measurement of the delay period. 1 supercooled.

. TABLEII Ignition delays of low'molecular weight mercaptale and mercaptols using low-freezing WFNA or a WFNA alkqnesulfonicacid blendasthe'oxidizer' I i a I lu'elfreaction product ofi Ignition Oxidizer Ten i p dela. Mercaptan (mrg' (tempo Carbonylcomponentnent Acetaldehydeun Low-freezingwFNA -40 100 Acetone "do -40 Formaldehyde -40 5, 000

1 Formaldehyde- 85% HNOa-15% Methane Sulfonlc Acid -40. r 18. 5 Em 1 Aceta1dehyde.. .do -40. 8 y Acetone do-. +40 11 Methyl Ethyl Ketone -d0 70 78 -.Ally 1.... Acetaldehyde 85% HNO3(99%)15% Methane sulfonic'Acliu. I -70l 46' Ethyl Acrolein 85% HN03-15% Methane Sultonlc Acid 70 100 lLow-ireezlng WFNA comnn om Enos, 4% NeNo, and 4% 11,0.

' As. indicated. .above, a mixture. of. the m'ercaptol formed from ethyl mercaptan and acetone and the mercaptal formed from ethylmercaptan and acetalde-f hyde, i.e;,' a; mixture of-isopropylidene diethyl mercaptol and ethylidene diethyl 'mercaptal is anzespecially eflicient reducing. componentof a hypergolic mixture suitable for use in thrust engines. The properties of the mercaptal, the. mercaptol and the. blend are set forth in the follow- 7 ing Table III:

TABLE HI Some physical properties of ethyl mercaptol of acetone (isopropylidene' diethyl mercaptol), ethyl mercaptal of acetaldehyae (ethylidenediethyl mercaptal), and a 75-25 blend of the mercaptol and mercaptal eaptol and mercaptal-has' an average. ignition delay period of: 26.5 mil1isjeconds,. i .e,, substantially stable-liquid at -70- F. I 5.

The.mercaptalsandmercaptols oi the invention term that. of the" nie'rcaptal: component andthe blend is a hypergolic. mixtures withfconcentrated nitric acid; Commercial nitric. acidfhaving .a. purity. the. range. from 9.5 to.10.0.%. NHO .by weight isasatisfactory oxidizinggagent. White fuming: nitric acidtcomposed of 98.5% byweight HNO and 1.5%v by weight: ofwat er is an. effective oxidizing agent. Minor proportions-of ignition. acceleratorsor freeze pointv depressants may be addedeto. the nitric acid which is mixed with. thev mercaptals. and' rnercaptolsv to cause. spontaneous'ignitionof themixture. For example, 2. to 20% by .weight of-nitrogendioxide maybe dissolved inthe nitric acid. This mixture. is. sometimes referredto as. red. fuming nitric acid. 2. to 20%. by weight sulfuric acid may be added. to. the nitric. acid to. accelerate ignition. .240 2.0% ofn'itrosyl. sulfuric acid. may. beadded to the nitric. acid as. an. ignition. accelerator. '5. to. 25%; by 'weightof a. lower. alkane sulf'onic acidsuchas. methane ,sulfonic acid, ethane. sulfonic acid, ancL the like, may-be added to the nitric acid to produce a mixture characterized by a much reduced freeze. pointand good ignitionproperties whenlmixed' with the'. mercaptalsfand mercaptols. Nitric acid: containing .a smalllamount up'toiabout.4% by we? 9 $9 ll 9y iil aqsi eanel m n-. 11. t bet 4% by weight of water has a low freeze point and forms hypergolic mixtures with the mercaptals and mercaptols of the invention.

If the mixtures of nitric acid and mercaptals and metcaptols are to be employed as a thrust engine fuel, it is desirable that the mixture contain approximately the quantity of nitric acid stoichiometrically required for complete oxidation of the organic material to water, carbon dioxide and sulfur dioxide. Maximum thrust appears to be obtained if the nitric acid is present in the mixture at about 85 to 95% of the amount stoichiometrically required for complete oxidation. Where it is simply desired to em ploy a mixture of mercaptal or mercaptol with nitric acid to start a fire, as, for example, when such a mixture is introduced into the bottom of an oil well to initiate burning in the formation, the proportions of the two components may be varied rather widely, for example, mixtures containing the organic compound and the nitric acid at weight ratios from 1:5 to 5:1 may be employed.

We claim:

1. The method of starting a fire by auto-ignition, which comprises mixing a material having the formula:

wherein R and R are materials selected from the group consisting of alkyl radicals and alkenyl radicals and R" is a material selected from the group consisting of alkyl radicals, alkenyl radicals and hydrogen and wherein the total number of carbon atoms is in the range 3 to 12 with suflicient concentrated nitric acid to form a mixture containing said material and nitric acid at a weight ratio within the range 1:5 to 5:1. 1

2. A fuel characterized in that it ignites spontaneousl when mixed with fuming nitric acid consisting essentially of about 75% by weight of isopropylidene diethyl mercaptol and about 25% by weight of ethylidene diethyl mercaptal.

References Cited in the file of this patent Hackhs Chemical Dictionary, 3rd edition(1944), The Blakiston Co., Philadelphia, pages 598, 599.

The Van Nostrand Chemists Dictionary, D. Van Nostrand Co., Inc., N.Y. (1953), pages 458, 459. 

1. THE METHOD OF STARTING A FIRE BY AUTO-INGNITION, WHICH COMPRISE MIXING A MATERIAL HAVING THE FORMULA: 